1. Field of the Invention
The present invention relates to a system and method for actuation of a valve, such as an intake and/or exhaust valve of an internal combustion engine.
2. Background Art
Conventional internal combustion engines use a camshaft to mechanically actuate the intake and exhaust valves of the cylinders or combustion chambers. The fixed valve timing of this arrangement, or limited timing adjustment available for variable cam timing systems, limits control flexibility. Electronic valve actuation (EVA) offers greater control authority and can significantly improve engine performance and fuel economy under various operating conditions. Electromagnetic actuators are often used in EVA systems to electrically or electronically open and close the intake and/or exhaust valves.
Electromagnetic actuators controlled by an associated valve controller, engine controller, and/or vehicle controller may use electromagnets or solenoids to attract an armature that operates on the valve stem. In a typical electromagnetic actuator, two opposing electromagnets and associated springs are used to open and close an engine valve in response to the signals generated by the controller. The upper and lower electromagnets are energized to assist the springs in closing and opening the valve, respectively, and to hold the valve closed or open against the associated spring force. The upper spring exerts a downward force that pushes the valve downward as the upper electromagnet is turned off, while the lower spring exerts an upward force that pushes the valve upward as the lower electromagnet is turned off. The opening, closing and landing speeds of the valve are functions of a number of parameters including the spring forces and the excitation currents of the electromagnets.
For many applications it is desirable to provide fast, controlled valve actuation to improve engine performance without a significant increase in actuator power consumption, which could adversely affect fuel economy. Power consumption is affected by the speed with which current is removed from the electromagnets when releasing the armature. During release of the armature from either the upper or lower electromagnet, current to the holding electromagnet should stop quickly. Otherwise, mechanical potential energy stored in the associated spring is not converted into motion, but instead into electrical energy that must be recycled through the associated electronic circuitry, with an inevitable loss. If excessive spring energy is converted to electrical energy during the launch because of slow current quenching, the spring/armature system may not have sufficient kinetic energy to reliably move the armature within the catching region of the opposing electromagnet during the subsequent valve landing to be reliably caught.
Similarly, energy supplied to the new holding coil (or catching coil) should be controlled and supplied at a rapid rate at the appropriate time to avoid electrical resistive losses during flight while still providing controlled and reliable valve landings for repeatability and durability.
Prior art EVA control strategies have incorporated one or more capacitors in the control circuitry for energy recovery. For example, Japanese patent application 10-282974 (Pub. No. 2000-110593) published Apr. 18, 2000 discloses the use of capacitors to store energy released during shut off of a coil Lo power the same coil and/or an alternate or following coil during a subsequent energization. Similarly, U.S. Pat. No. 3,896,346 discloses a parallel or shunting capacitor to store energy recovered from one coil during de-energization to subsequently energize another coil.
Some prior art EVA control strategies have employed dual “H” bridges to separately control the two electromagnets to control valve movement. Using “H” bridges without any other associated energy storage makes power supply voltage selection difficult. If low power supply voltage is selected, the low voltage would need to be applied for a considerable period of time before holding coil magnetic energy was removed and valve motion could begin. This limits valve timing control flexibility because the control action must be determined long before actual valve motion. Furthermore, because valve motion would begin with a considerable current in the holding coil, and current would remain longer because of the low voltage, considerable conversion of mechanical to electrical energy could occur during launch. In addition, the electrical energy needed for holding would need to be inserted into the attracting coil for a longer time while also inserting energy needed to compensate for losses to friction and gas forces resulting in large coil currents and high resistive losses. Although a high voltage supply could be used to apply a high voltage for a short period of time to remove holding coil energy and add the needed holding coil energy, the high voltage supply is needed only for a short time during the launch and landing phases of armature motion. However, complex circuitry to control the high voltage supply would be present at all times. As such, selection of either a high or low voltage supply with conventional “H” bridge circuitry results in wasted energy, because regenerated energy and current flows backward through various “H” bridge components to the power supply when reverse voltage is applied to the holding coil during launch. In addition, such an arrangement requires additional “H” bridge components to allow applied coil voltage to be reversed.